Porous ceramic, polymer and metal materials with pores created by biological fermentation

ABSTRACT

Porous polymers are made by adding biologically active agent and growth substrates (e.g., yeast and sugar, preferably in the presence of water or other suitable fluid) to a polymer forming material, which may be a liquid. The yeast acts on the sugar, forming carbon dioxide gas bubbles. The material is then polymerized so that the gas bubbles create permanent pores within the polymeric material. The polymer can be an epoxy for example. The pores will contain residue of the yeast. Also, porous metals can be made by combining a metal powder with yeast, sugar, and water. The porous metal paste is then sintered. Porous ceramics and semiconductors can be made by combining the yeast and sugar with a ceramic forming liquid such as polysilazane. Polysilazane converts to silica when heated, which helps to bind the ceramic or semiconductor powder particles at a reduced temperature. Biological agents other than yeast (e.g. bacteria, enzymes), and growth substrates other than sugar can also be used.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/885,488 filed Jul. 7, 2004, now U.S. Pat. No. 7,157,115, and thecomplete contents thereof is herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to methods for creating porousmaterials. More specifically, it relates to a method for creating poresin materials by biological fermentation (e.g. with bacteria or yeast orenzymes).

BACKGROUND OF THE INVENTION

Porous ceramic materials have been made previously by adding organicmaterials to the ceramic during fabrication, and then burning out theorganic materials to leave holes or voids therein. For example, in brickmanufacturing, sawdust or wood powder (i.e., “fugitive material”) hasbeen added to make the bricks lighter. On firing, the sawdust burns out,leaving a void where the wood once resided. This methodology has anumber of drawbacks. In particular, it requires a significant amount ofadditional energy input (e.g., heating of the ceramic to combust thesawdust) and time. If the bricks are fired too fast, burning sawdust cancause the bricks to fracture. In addition, the burning process can leavea significant amount of residue to remove from the resulting porousceramic material depending on the time and temperature of heating.Finally, the fugitive material adversely impacts profitability for themanufacturer by requiring space in the plant to store the materialbefore use. This involves both an expense for storage and a decrease inthe production capacity of the plant since some of the area is used upfor storage purposes.

Porous polymeric materials can be made by a process wherein hollow glassspheres are combined with the polymer forming material. The density ofthe polymer composite can be varied by varying the density of the glassspheres and the volume fraction of spheres added. However, similar toprior porous ceramic structure manufacturing techniques, techniques forforming porous polymeric materials suffer from the fact that a verylarge volume of usable plant space must be reserved to store the largevolumes of the glass spheres. This usage of space for storage of theglass spheres carries a very high cost both in overhead costs and inlost production capacity.

U.S. Pat. No. 5,071,747 to Hough et al. describes porous polymericmaterial, which in one embodiment, includes yeast within the pores ofthe material. The Hough invention contemplates formation of an emulsionfrom monomers and pre-polymers, followed by polymerizing the monomersand pre-polymers to yield a porous polymeric material, and finally,incorporation of the yeast within the pores. The yeast does not functionto create the pores in the Hough device. Rather, the yeast providesbiological activity in the device that is ultimately produced.

U.S. Pat. No. 4,603,111 to Keller describes a process for making apolyacrylamide bead, which in one embodiment incorporates yeast. Theprocess involves combining the yeast with the acrylamide monomers,followed by polymerization. The resulting product is a bead with yeastimmobilized thereon and therein. It was determined that the immobilizedyeast retained the same activity of non-immobilized yeast. Thus, thebeads could then be ideally used in later processes. It is noted thatthe yeast in Keller do not function in any capacity to form pores in thepolyacrylamide bead.

U.S. Pat. No. 5,705,118 to Hayes describes a process for forming aceramic material which includes combining an organic material such asgluten with a ceramic, followed by firing the ceramic material to form agreen body. The gluten functions as a binder, and is ultimatelyeliminated by heat treatment in the manner discussed above inconjunction with prior technologies. Hayes indicates that minor amountsof yeast or enzymes, as well as many other constituents, may be includedwith the gluten, and that a “risen loaf” from the ceramic/gluten/yeastor enzyme mixture is ultimately fired.

U.S. Patent Publication 2003/0171822 to Lo describes a process forcreating a porous synthetic bone graft wherein ceramic powder, binderand a pore-forming agent are combined in an inert liquid. The poreforming agent is then allowed to create the pores, and the porousstructure is then fixed by a heat treatment. A high temperature heatingis then used to eliminate the binder and the pore forming agent, and tofuse the structure together. Lo suggests that the pore forming agentscould be yeast cells, alkali metal salts, and inorganic salts of acidsderived from carbon and phosphorous. Lo indicates that a carbohydratepowder, and, in the case of yeast being used as the pore forming agent,sugar, are added to the slurry. Lo does not contemplate combining metalmaterials with the ceramic. Rather, in Lo, it is important to have poresin order to allow osteoblasts to attach in order to promotemineralization. Further, the Lo process methodology is not applicable topolymer materials as it requires the use of a binder, and a post poreforming, ceramic fusing heat step. Likewise, the Lo process suffers fromthe requirement of using carbohydrate powders and binders, which mayresult in porous structure where the pores are less uniform in sizeand/or are larger in size than is desired for certain industrialapplications as opposed to applications in the human body.

It would be an advance in the art to provide a method for making porousmaterials that does not require large volumes of fugitive material. Itwould also be an advance to provide methods for making porous materialsthat can be used with metals, semiconductors, and polymers.

SUMMARY OF THE INVENTION

The present invention includes a porous polymeric material having closedpores, wherein the closed pores contain a biologic agent or a residue ofbiological agent. The residue can be a decomposition or desiccationproduct of yeast, bacteria or enzyme, for example. Carbon dioxidegenerated by the biological agent can also be trapped within the closedpores, but may also diffuse through the polymer material over time. Thepolymeric matrix material can be epoxy or a variety of other polymers orcombinations of polymers.

The present invention also includes a method for making a porouspolymeric material. In the method, a polymer forming material is mixedwith a biological agent (e.g., yeast), and a growth substrate (e.g.,sugar). The biological agent is allowed to act on the growth substrateand form gas bubbles. Then, the polymer forming material is polymerized,and in the process of polymerization, it traps gas bubbles therein. Thesize of the bubbles will vary depending on the polymer forming materialsused, rate of polymerization, and other processing conditions. Thebiological agent can act on the growth substrate at the same time thatthe polymer forming material is polymerizing.

The present invention also includes a method for making a porous ceramicor porous semiconductor material. In this method, a powder of ceramic orsemiconductor material is mixed with a biological agent (e.g., yeast), agrowth substrate (e.g., sugar), and a ceramic forming liquid binder(e.g., polysilazane). The biological agent is allowed to act on thegrowth substrate to form gas bubbles which separate powderized ceramicor semiconductor material except at particular points on the surfaces ofthe powder. The binder may hold the powderized ceramic or semiconductormaterial together at the points of contact. Then, the mixture is heatedso that the ceramic forming liquid is converted to an oxide material,thereby binding the ceramic or semiconductor particles together. Thematerial may be further heated to sinter the ceramic or semiconductorparticles together.

The present invention also includes a method for making a porous metalmaterial. In this method, a powder of metal material is mixed with abiological agent (e.g., yeast), and a growth substrate (e.g., sugar).The biological agent is allowed to act on the growth substrate to formgas bubbles. Then, the mixture is heated to bond the metal particlestogether. The metal particles, depending on the metal particles chosen,may also be sintered by further heat treatment.

The present invention also includes a porous ceramic with embedded metalwires. The ceramic can be made of many different ceramic materials, andthe metal wires can be made of many different metal materials. Theceramic can be dried or baked at relatively low temperature, or may besintered at relatively high temperature.

DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 shows a porous polymer material made according to the presentinvention.

FIG. 2 shows a porous ceramic with metal wires according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The present invention provides several methods for making porouspolymers, metals, semiconductors and ceramics.

For making porous polymers according to the present invention, polymerforming material (e.g., monomers or oligomers) or an already formedpolymer is mixed with a combination of biologically active material(preferably yeast, but also may include bacteria or enzymes), a growthsubstrate (preferably sugar, but also may include polysaccharides,mixtures of sugars, and other materials which can be acted upon by thebiologically active material to produce a gaseous byproduct), andpreferably a fluid carrier (e.g., preferably water, but any fluid whichallows mixing of the polymer forming material or polymer, biologicallyactive material and growth substrate, and which does not inhibit theactivity of the biologically active material would be acceptable). Inthe most preferred embodiment, yeast, sugar and water are used incombination with a polymer forming material. In this embodiment, theyeast is allowed to ferment the sugar to produce carbon dioxide bubblesin the polymer as the polymer forming material is polymerized.Polymerization can proceed by a variety of different mechanismsincluding without limitation condensation or step-type polymerizations,free-radical polymerizations, ionic and coordination polymerization,ring opening polymerization, and photolytic or electrolyticpolymerization. Polymerization can be initiated by a variety ofdifferent mechanisms including addition of a catalyst, addition of oneor more reactants, application of radiant energy (e.g., UV radiation),and application of heat. The polymer formed can be aqueous (i.e.water-soluble) or non-aqueous (immiscible in water).

In some embodiments of this invention, such as where the polymer iswater soluble, or under conditions where the polymer is of a consistencythat allows water droplets carrying the biologically active material andthe substrate to be dispersed therein, a preformed polymer can be used.

In another aspect, a ceramic or semiconductor powder (e.g., silicon) iscombined with a ceramic forming liquid binder (e.g., polysilazane,alumoxane, etc.), and a combination of biologically active agent, agrowth substrate, and preferably a fluid (e.g., yeast, sugar and water,respectively). After the yeast ferments and produces gas bubbles in themixture, the material is heated to sinter the ceramic or semiconductorto form a porous material. The ceramic forming binder converts to anoxide, carbide or nitride during the sintering step (e.g. polysilazaneconverts to nitride or silica depending on temperature).

In another aspect, a ceramic powder is combined with metal wires, and acombination of biologically active agent, a growth substrate, andpreferably a fluid (e.g., yeast, sugar and water, respectively). Anoptional binder, such as wheat gluten, wheat flour or polysilazane mayalso be added. After sintering, the metal wires remain in the porousmaterial and provide unusual and useful properties heretofore notavailable.

In another aspect, a metal powder (e.g. stainless steel, copper, bronze,zirconia) is combined with a mixture of a biologically active agent, agrowth substrate, and preferably a fluid (e.g., yeast, sugar and water,respectively). The yeast produces pores in the material, and the metalpowder is then sintered, resulting in a porous metal material.

Porous Polymer Materials

Porous polymeric materials are preferably made in the present inventionby mixing a polymer-forming material (e.g. monomers, oligomers, andreactants and/or catalysts) or an already formed polymer together with acombination of yeast, sugar and water (and optionally salt and othernutrients). After mixing, the mixture is left to stand so that the yeastconsumes the sugar and produces carbon dioxide. The carbon dioxide formsgas bubbles that remain trapped within the polymer formed from thepolymer forming material. The polymer forming material may bepolymerized before, during or after the yeast causes carbon dioxidebubbles to be produced. The polymer material may trap the carbon dioxidebubbles thus formed within the porous material. However, depending onthe nature of the polymer employed, the carbon dioxide may either leachout of the porous material over a period of time, or be trapped withinthe polymer in gas pockets for an extended period of time.

The polymer or polymer forming material can vary widely within thepractice of the invention and will depend on the needs of theapplication. For example, the polymer forming material can be a 2-partepoxy, 1-part epoxy, polyester resin, silicone, phenolic, rubber or anyother polymerizable material. The polymer can be a hard polymer (e.g.,phenolic or epoxy), or a soft elastomer (e.g., rubber or silicone). Thepolymer forming material will typically be a liquid until it ispolymerized. In applications involving an aqueous polymer, the polymericmaterial will be present in the mixture in already polymerized form, andthe gas formation caused by the yeast and sugar will take place as thewater is evaporated from a mixture containing the aqueous polymer.

The relative proportions of polymer forming material, yeast, sugar andwater can vary widely. The relative proportions can be selected toproduce a desired porosity or pore connectivity (e.g., connected poresor non-connected pores (i.e., singular, un-connected voids within thepolymeric material)). For example, the polymer forming material cancomprise about 40-80% by volume, with the sugar and water eachcomprising about 5-20% by volume, and the yeast comprising 1-10% byvolume. These ranges are exemplary only and are not limiting to theappended claims.

In order to provide uniform pore distribution, the mixture can becontinuously mixed while the yeast acts upon the sugar. Mixing cancontinue until the mixture reaches a predetermined viscosity. Forexample, mixing can be halted when the viscosity is high enough toassure that the bubbles will not rise and separate from the polymericmaterial. Mixing until high viscosity is achieved will tend to reducethe size of the bubbles and possibly elongate the bubbles into longtubes which may be desirable in some applications.

Preferably, the mixture has a sufficiently high viscosity to prevent gasbubbles from rising without continuous mixing. Optionally, in order toprovide a high enough viscosity the material can be partiallypolymerized before or during gas generation (e.g., yeast or bacterialfermentation, or enzymatic reaction). Partial polymerization tends toincrease the viscosity and inhibit separation of the bubbles from thepolymer.

Many different biologically active agents other than yeast can be used.For example, bacteria or viruses can be used. The biological agent mustform a gas as it consumes and/or interacts with the growth substrate(e.g., sugar) in the mixture.

The biological agent may need to be selected for compatibility with thepolymeric material. For example, some polymer-forming materials might betoxic to certain bacteria or yeasts. It is essential in the inventionthat the biological agent remains metabolically active while exposed tothe polymer-forming material.

Many growth substrates other than sugar can be used. For example,carbohydrates, fats, proteins, mixtures thereof or other substratematerials (e.g., wheat flour) can be provided in the mixture. Whatevergrowth substrate is provided, the biological agent must be able toconsume and/or interact with it and produce bubble-forming gases.Nutrients such as salt, minerals or B-vitamins can also be added toenhance the generation of bubbles or to provide improved environmentalconditions for the biological agent.

The polymer forming material can be water-soluble (e.g., (PAM)polyacrylamide, methylcellulose, or (PVA) polyvinyl alcohol) orwater-immiscible (e.g., epoxy, polyester resin, polyamides, orpolyimides). If the polymer forming material is water soluble, then theyeast, sugar and water will be wetted and uniformly distributed withinthe material after mixing. If the polymer forming material iswater-immiscible, then the yeast, sugar and water will form tinydroplets within the polymer-forming material. In either case, mixingmight be desirable to encourage the uniform distribution of gas bubbles.

The polymer forming material can be polymerized by any known method. Thepolymer forming material can be polymerized in the presence of heat orradiant energy, or by the addition of a chemical catalyst, for example.If a chemical catalyst is used, it can be incorporated into the mixturebefore or after the gas bubbles are formed. In some applications, thepolymerizing step can inactivate or kill the biological agent (e.g., UVexposure is known to kill a variety of organisms; heat also can kill avariety of organisms; further the choice of polymeric material mayresult in surface interactions that kill the biological agent).

The porosity of the porous polymer produced can be controlled andadjusted by a number of mechanisms. For example, the porosity might becontrolled by controlling the duration or temperature of thefermentation, or by adjusting the quantity or composition of the growthsubstrate. The present porous polymer material can have a wide range ofporosities, for example up to 50 or 75%. Typically, some of the pores inthe material will be open pores (i.e., open to the external environment)and some will be closed pores (i.e., isolated from the externalenvironment).

The present method for making porous polymeric materials is suitable formaking molded parts or other shaped items. After mixing the materials,and before or after the formation of bubbles, the bubbly mixture can bepoured into a mold, extruded through a die, or otherwise shaped.

The porous polymers made according to the present invention willnecessarily have either the biological agent itself or a residue of thebiological agent trapped within the closed pores. The open pores may ormay not contain similar agent or residue. The residue may comprise dead,dried or decomposed yeast, bacteria, or inactive or active enzymes usedin combination with the growth substrate to generate the gas bubbles.The closed pores may also contain a portion of unconsumed growthsubstrate or nutrient materials.

The closed pores might also contain water. The water might be left overfrom the original mixture (e.g., from the yeast, sugar and watermixture), or might be created by polymerization. Also, it is possible insome polymer materials for the water to slowly diffuse through thepolymer and open pores, so that the closed pores contain only driedbiological agent and growth substrate residue.

FIG. 1 shows a porous polymeric material according to the presentinvention. The porous material comprises a solid polymeric matrix 20.Gas-filled and/or empty pores 22 are disposed within the polymericmatrix. The pores 22 typically will have many different sizes, forexample in the range of 0.1 micron to several millimeters. The pore sizewill typically depend on the processing (e.g., mixing or agitation) ofthe polymeric material during polymerization or on the reactionconditions and the amounts of biologically active agent and growthsubstrate provided. Although the pores are shown as spherical, the porescan be any shape.

The pores 22 can be open or closed to the outside environment.Typically, some pores will be open, and some pores will be closed. Pores22 a are open to the outside environment. In the present invention, theclosed pores (i.e., pores with trapped contents) will contain thebiological agent or a residue of biological agent 26. The residue 26 cancomprise any decomposition or desiccated product of a biological activeagent (e.g., bacteria, yeast, enzyme).

The present porous polymer material is well suited for use inapplications where lightweight polymer materials are needed. Also, thepresent porous polymer tends to reduce the amount of polymer materialneeded to fabricate a part with a given volume, which can reduce thecomponent cost.

EXAMPLE 1

In this experiment, the biological agent was commercially availablebaking yeast. The polymer system chosen for demonstration was a commontwo-part epoxy—Epoxy 907, commercially available from Miller-Stephenson.Equal parts of the two-part epoxy were used. In one example,approximately 15 grams of each epoxy part was used, along withapproximately 2 grams of yeast, 5 grams of sugar and 5 grams of water.The yeast, water, sugar, and epoxy were mixed together in a beaker. Insome cases 1 gram of salt was used as well, and the yeast, sugar, water,and salt were mixed in a beaker first and the yeast mixture was allowedto stand for a few minutes. This mixture was then added to the epoxy andmixed. After mixing for a short period of time, the epoxy was allowed torise in a mold. The article was then observed by optical microscopy, andthe densities characterized by Archimedes method. Pore sizes were in therange of approximately a few millimeters to tens of microns. Porositywas up to about 50% by volume.

Samples of the porous epoxy material were exposed to water and the waterwas not absorbed. This was most likely due to the surface tension of thewater in contact with the polymer and the hydrophobic nature of theepoxy. It was possible that the pores, which existed in the specimen,were “closed”, such that the fluid could not be absorbed into the poresjust below the surface of the specimen. To test, alcohol was placed onthe sample, which was quickly absorbed. This indicated a significantamount of open porosity was present in the sample. To determine theamount of total porosity, percent open porosity and percent closedporosity, a modification of Archimedes method was used with alcohol asthe fluid medium.

A sample was cut into a parallelogram, weighed dry, placed in alcohol tosaturate, weighed saturated, and weighed suspended. Based on theseweights, the sample was calculated to contain approximately 30% byvolume open porosity, 20% by volume closed porosity (i.e., 50% totalporosity). The pores, which varied in size from a few mm to the micronrange or below, was random in both size and spatial distribution.

Porous Metal Method

The present invention includes a method for making porous metals andmetal foams in which a metal powder is mixed with a combination ofbiological active agent (e.g., yeast), a growth substrate (e.g., sugar),and fluid (e.g., water). As above, the biological active agent can beany of yeast, bacteria or enzyme that consumes or interacts with thegrowth substrate so as to produce gas bubbles. The substrate can be, forexample, any sugar, carbohydrate, fat, protein or mixture thereof.

In a manner similar to producing porous polymers, the porous metalmaterial is made by mixing the metal powder with, for example, theyeast, sugar and water to form a slurry or paste. The yeast, or otherbiologically active agent, is then allowed to act on the growthsubstrate to produce, for example, carbon dioxide gas in the case ofsugar and/or other polysaccharide materials being used as the growthsubstrate. As the yeast produces gas, bubbles are formed in the mixture.After bubbles are formed, the mixture is heated to bond or sinter themetal powder particles. The yeast and sugar might or might not be burnedout during the heating step; some yeast or sugar may remain in the finalproduct.

The metal powder can be made of many different metals or metals alloys.For example, the metal powder can be made of aluminum, copper, stainlesssteel, brass, bronze, or other metals.

Optionally, a binding agent is also provided. The binding agentfunctions to hold the metal powder together as it is heated to sinteringtemperature. The binding agent can be an ceramic forming liquid (e.g.,polysilazane, alumoxane, etc.), methylcellulose, acrylic, soluble orinsoluble fiber or other polysaccharide gums, or wheat flour or gluten.Alternatively, the binding agent can be a low-melting point metal (e.g.,tin, lead, bismuth, or zinc) that may hold the metal powder together bysurface tension. The low melting point metal can be provided in the formof a powder.

In some applications, it may be necessary to perform the heating orsintering step in an oxygen-free or reducing atmosphere (e.g., innitrogen, hydrogen, or argon).

In another aspect of the invention, the porous metal material includesmetal wires. The wires can have a small diameter (e.g. tens or hundredsof microns). The wires can have short lengths of several millimeters orseveral centimeters. The wires tend to strengthen the porous metalmaterial by distributing tensile and compressive forces. The wires canbe made of stainless steel, for example.

Additionally, the porous metal material can include a portion of aceramic powder.

Porous Ceramic or Semiconductor Method

The present invention also includes porous ceramic or semiconductormaterial made with a ceramic forming liquid binder such as polysilazane,alumoxane, or silicate. In this aspect of the invention, the porousstructure is formed by a biological active agent acting on a growthsubstrate, as described above. The biological active agent can be anyyeast, bacteria or enzyme that consumes or interacts with the substrateto produce gas bubbles. The substrate can be any sugar, carbohydrate,fat, protein or mixture thereof.

Polysilazane and alumoxane convert to oxide, carbide or nitride ceramicmaterials when heated. Silicates that form silica can also be used asthe ceramic forming material. The ceramic material formed from thebinder liquid tends to assist in bonding the ceramic or semiconductorparticles together. Therefore, the ceramic forming liquid binder canreduce the temperature required for hardening or sintering the material.This can be a great advantage because sintering at high temperature canbe very energy consumptive and expensive. By reducing the temperaturerequired for bonding, and/or reducing the high temperature firingduration, the ceramic forming liquid can greatly reduce the time andcost required to make porous ceramics and porous semiconductormaterials.

In the case of ceramic, the ceramic powder can comprise many differentceramics such as zirconia, alumina, silica, silicon nitride siliconaluminum oxynitride, silicon carbide, clay, porcelain, mullite,codierite, and the like.

In the case of semiconductor, the semiconductor can comprise manydifferent semiconductor materials including silicon, germanium and thelike. Also, compound semiconductors can be used (e.g., 3-5 or 2-6materials). The use of ceramic forming liquids for producing porouscompound semiconductors can be advantageous in the case of compoundsemiconductors because some compound semiconductors cannot withstandhigh temperatures required for sintering.

Also, it is noted that the present invention contemplates a porousmaterial comprising mixtures of ceramics and semiconductors, with theparticles adhered by the decomposition product of the ceramic formingliquid binder.

Additionally, it is noted that many different kinds of ceramic formingliquids can be used. Additionally, it is noted that the ceramic formingliquid can be diluted with a solvent to control the amount of oxidematerial formed during baking.

Semiconductor Powder Example

Silicon Powder (ground with a mortar and pestle to a fine consistency)—1gram;

Polysilazane (binder)—1 gram

Yeast 5 grams

Water 15 grams

Sugar 10 grams

The yeast, sugar and water were added to a separate beaker and mixed.This mixture was set aside to allow the yeast to activate. The siliconmetal powder was mixed with the polysilazane thoroughly. The yeast andsilicon mixtures were then combined. The resulting mixture was formedinto two small pellets and heated gently with a heat gun to set thebinder.

Porous Ceramic with Metal Wires

The present invention also includes a porous ceramic material comprisingembedded metal wires. Preferably, the porous ceramic is made by abiological active agent acting on a growth substrate, as describedabove.

FIG. 2 shows a ceramic 30 with embedded metal wires 32 according to thepresent invention. The ceramic is porous and has pores 22. The wires 32can have diameters of tens or hundreds of microns and lengths of severalmillimeters or several centimeters. The wires can be made of stainlesssteel, copper, aluminum brass, bronze, titanium, tungsten, nickel orother metals for example. The wires can comprise less than 1% or morethan 5%, 10% or 20% of the composite material volume, for example.

The ceramic powder can comprise many different ceramics such aszirconia, alumina, silica, silicon nitride, silicon aluminum oxynitride,silicon carbide, clay, porcelain, mullite, codierite, and the like.

The embodiment with metal wires can be made by adding chopped metalwires to a mixture of ceramic powder, yeast (or other biological agent),sugar (or other growth substrate) and water (or other fluid). After thefermentation step, the material is dried, baked or sintered. Sinteringor heating is not necessarily required for creating a hard and durablematerial. If the material is sintered at high temperature, it ispreferable for the sintering temperature of the ceramic to be less thanthe melting temperature of the metal wire. The particle size of theceramic material may be selected to be small so that the ceramicmaterial sinters at a temperature lower than the wire meltingtemperature.

Porous composites comprised of metallic wires and ceramic lattice mayhave many potential uses. Reducing the density of a ceramic material, ingeneral, reduces the strength and toughness of the material, as is wellknow in the field. In some cases, very low-density materials can be made(for example, 95% by volume air), but in general, these low-densitymaterials are difficult to handle because they are brittle and are oflow strength and toughness. Addition of metal wires will improve thestrength and toughness of these materials. For example, porous thermalinsulation materials, filtration media, etc., may be envisioned withincreased strength and toughness. This might benefit a wide array ofapplications, including tiles for space shuttles, refractory furnaceinsulation, filtration media (air, water and molten metal), etc.

Also, the wires 32 can provide electrical conductivity to the porousceramic, and can absorb or block electromagnetic radiation or certainfrequencies of electromagnetic radiation. Also, the wires can improvethe thermal shock resistance of the part, which may be very beneficialin applications such as kiln furniture or insulating tiles. In thisapplication, the wires are preferably made of a metal with a highthermal conductivity (e.g. copper).

Metallic materials of various types, such as platinum, rhodium, etc, areoften used in catalytic applications. Many of these materials are veryexpensive which prohibits manufacturing and use of monolithic pieces inthe catalytic applications. Instead, ceramic supports are often used as“substrates” to hold the film or coating of the precious metal material.Utilizing a porous composite material which incorporates metallic wires,may allow improve catalytic efficiency per unit weight of the catalyticmaterial used. The substrates, which are coated with the precious metal,only allow interaction of the material to be reacted with the topsurface of the catalyst. The bottom surface of the catalyst is incontact with only the substrate and not the reacting gases. Essentially,the bottom surface is wasted as far as the catalytic reactions areconcerned. However, utilizing a highly porous ceramic substrate intowhich the “wires” of the catalytic material are incorporated may allowreaction of the material around the entire circumference of the metalwire. The critical parameters, which can be varied to tune the catalyticactivity, include the amount of porosity produced, the size rangedistribution of the porosity, the diameters of the wires used and thelength of the wires. Also, tailoring the parameters of forming, such asthe “wetability” of the metal wires by the ceramic phase, can dictatewhere the wires occur in the composite. Also, tailoring the surfaceproperties of the metal wires (for example by coatings), can dictatewhere the yeast cells multiply and where they will be killed. Forexample, if the sugar necessary for the yeast cells to multiply iscoated onto the wires before addition to the mixture, it may be possibleto preferentially grow the yeast cells around the wires. Likewise, afungicide applied to the wires may be able to retard growth of the yeastcells in the vicinity of the wires. Upon heating, the fungicidalmaterial could be removed if desired. In some applications, however, itmay be advantageous to leave the fungicidal, germicidal or disinfectingagent within the porous material, which may produce parts that do notallow growth of these agents. Many schemes may be envisioned to tailorhow the porosity forms in relation to the wires and this may dictate thecontrol of the degree of surface area of the fibers available for thecatalytic reactions.

The function of the ceramic material is to provide the substrate forholding the fine diameter wires. Many catalytic operations are performedat high temperatures and in relatively corrosive environments where theceramic materials can provide both refractoriness and inertness. Themetallic wires serve as the catalytic sites. The yeast, which producesthe porosity, serves to open the interior of the part to the catalyticreactions and allow the gases and/or liquids to permeate into theinterior of the part through the large amount of open porosity present.Therefore, the entire volume of the porous part is available for thecatalytic reactions. Also, incorporating the wires into the compositewill eliminate the need to coat the substrate with the precious metal(which is a common current method) after formation of the substrate. Thecoating process is costly, time consuming and in many cases is difficultto control to ensure uniform coating thickness throughout the parts.Utilizing metal wires may eliminate these problems, by incorporating thecatalytic material directly into the part during fabrication, and byusing controlled diameter wires.

Metal Powder/Ceramic Powder Composite Example

Ingredients: Stainless Steel chopped wires 1.5 gram

-   -   Zirconia Powder 2.0 gram    -   Duramax binder 1.2 gram    -   Water 0.1 gram    -   Sugar 0.4 gram    -   Yeast 0.2 gram

The ingredients were mixed together and formed into two balls and asmall flat disk. The samples were allowed to dry for several days afterwhich a porous metal, ceramic, polymer composites was formed.

It will be clear to one skilled in the art that the above embodiment maybe altered in many ways without departing from the scope of theinvention. Accordingly, the scope of the invention should be determinedby the following claims and their legal equivalents.

1. A method for making a porous ceramic or semiconductor material,comprising the steps of: a) mixing the following: a ceramic orsemiconductor powder, a microorganism or enzyme that can ferment asugar, a sugar, and a ceramic forming binder liquid selected from thegroup consisting of: polysilazanes, alumoxanes, and silicates, to form amixture; b) allowing the microorganism or enzyme to act on the sugar soas to produce gas bubbles in the mixture; and c) heating the mixture sothat the ceramic forming binder liquid is converted to a ceramicmaterial.
 2. The method of claim 1 wherein the ceramic material isselected from the group consisting of: oxides, carbides, and nitrides.